More regulation. Specifically requiring worker-owned means of production: re-organize the AI model companies as mutuals and mandate (by law) that their only customers can be individual-contributor-level workers or worker unions. Ditto with robotics.

Is there some way to disabuse people of the notion that higher taxes would help here? Taxes don't really affect entity size. In significant ways they do the opposite, because business expenses are deductions, so using profits for expansion becomes more favored over paying them out as dividends the more you increase the tax rate.

What you need here is antitrust enforcement and lower barriers to entry so that there are more companies that are each smaller, and correspondingly more millionaires and fewer billionaires.

Isn’t this basically the situation in Europe?

It seems like that just supercharges brain drain by incentivizing all the smartest and richest people to just move away to a place with lower taxes and less bureaucracy.